Mg-Al-Si ALLOY

ABSTRACT

A magnesium alloy containing 3.5 to 7% Al, 0.4 to 1.5% Si, up to 1% Mn and up to 2% Zn. Such alloy possesses the combination of high strength and improved creep resistance over the prior art alloys. This alloy is particularly adaptable to die casting.

United States Patent [1 1 Foerster 1 Feb. 27, 1973 [73] Assignee: The Dow Chemical Company,

Midland, Mich.

[22] Filed: June 5, 1970 [21] Appl. No.: 43,946

' Related u.s. Application Data [63] Continuation-impart of Ser. No. 687,457, Dec. 4,

1967, abandoned.

[52] US. Cl. ..-...75/168 M, 75/168 B, 164/168 C,

' 164/113 [51] Int. Cl. ..C22c 23/00, B22d 21/00 [58] Field of Search ..75/l68; 148/32; 164/113 [56] References Cited UNITED STATES PATENTS 1,946,069 2/1934 Grant et al. ..75/168 1,959,913 5/1934 Gann et al. ..25/168 3,067,028 12/1962 Foerster ..75/ 168 3,119,725 1/1964 Foerster ..75/168 FOREIGN PATENTS OR APPLICATIONS 27,994 1964 Japan ..75/168 711,677 1965 Canada ..75/168 1,555,251 1968 France ..75/168 OTHER PUBLICATIONS Primary Examiner-Charles N. Lovell AttorneyGriswold & Burdick, Stephen S. Grace and William R. Norris [5 7 ABSTRACT A magnesium alloy containing 3.5 to 7% A1, 0.4 to 1.5% Si, up to 1% Mn and up to 2% Zn. Such alloy possesses the combination of high strength and improved creep resistance over the prior art alloys. This alloy is particularly adaptable to die casting.

2 Claims, No Drawings MG-AL-SI ALLOY This application is a continuation-in-part of Ser. No. 687,457 filed Dec. 4, 1967 now abandoned.

BACKGROUND OF THE INVENTION Magnesium alloys containing silicon, manganese and high aluminum are useful in many structural applications due to their excellent strength properties. They also have good castability for die casting purposes. For example, Grant et al. (U.S. Pat. No. 1,946,069) discloses a magnesium die casting alloy containing 8-12 percent aluminum, 0.351.0 per cent silicon and 0.1-0.5 per cent manganese. Also, Dowmetal EX magnesium die casting alloy has 5.8-7.2 per cent aluminum and 0.1-0.3 per cent silicon. However, such alloys in elevated temperature applications, e.g., automobile engine parts, are subject to extensive creep after extended periods of time, finally leading to failure of the part.

Another magnesium alloy containing 0.5 to 3 per cent aluminum, 0.2 to 6 per cent silicon and 0.2 to 1 per cent manganese, is disclosed by Gann et al. (U.S. Pat. No. 1,959,913) as a forging alloy. However, this, even in the die cast condition, has lower strength and poorer castability than the high aluminum alloys.

A primary object of the present invention is to provide an alloy having high strength, good castability and art.

' THE INVENTION The present invention comprises an alloy consisting essentially of magnesium containing by weight from about 0.4 to 1.5 per cent silicon, from about 3.5 to about 7 per cent aluminum, up to about 1 per cent manganese and up to about 2 per cent zine; provided that when the silicon is present in the alloy in an amount less than about 0.7 per cent, manganese is present in an amount from about 0.1 to about 1 per cent. A preferred composition contains from about 4 to about 5.5 per cent aluminum, from about 0.6 to l per cent silicon, from about 0.2 to about 0.5 per cent manganese and up to about 1 per cent zinc.

In the practice of the present invention, conventional melting, alloying and die casting techniques, as practiced by those skilled in the art, can be employed, using alloying and base metal constituents containing nonnal amounts and types of impurities.

' The following examples are illustrative of the present alloy.

Various magnesium alloys were conventionally prepared, ladled into shot well of a die casting machine, and cast at about 1,200F into a panel box about 4.0 inches wide, about 7.0 inches long and about 1.0 inch high containing a center bossabout 6.0 inches long by 3/16 inch, high and wide. Samples were taken from the face of the panel on either side of the center boss and tested for ductility by per cent elongation (%E), strength properties by tensile strength (TS) and tensile yield strength (TYS) in thousand pounds per square inch (KS1) and creep resistance by per cent creep (%Creep) after 100 hours at 350F under 5,000 psi stress. The Table presents the results of these tests.

improved creep resistance over the alloys of the prior.

TABLE Compositionbal.Mg-% Example Al Si Mn Zn E TYS TS Creep A 8.1 0.77 0.13 0.45 0.5 22.4 27.3 1.90 B 6.4 0.28 0.35 0.45 2.6 19.2 28.7 2.43 C 6.0 0.59 4.1 20.0 30.0 2.34 D 4.0 0.61 5.1 18.0 28.8 2.04 l 6.0 0.72 0.26 0.48 3.1 19.6 29.2 0.92 2 5.9 1.10 0.26 0.42 2.8 21.3 2940.88 3 4.1 0.52 0.39 7.5 18.8 33.1 0.54 4 4.1 0.93 0.24 4.4 21.6 31.6 0.32 5 4.2 1.22 0.27 3.0 20.8 28.6 0.30 6 3.8 0.99 5.5 20.5 32.8 0.34

Of the Comparison Examples (Ex. A-D), Example A is similar to the alloy disclosed to Grant et al, Example B is similar to the Dowmetal alloy previously referred to, and Examples C-D are alloy compositions with silicon less than 0.7 per cent and no added Mn. Example A, outside the scope of the present alloy because of the 8% Al content, has high strength but relatively low creep resistance. Example B, an alloy with lower aluminum, (outside the scope of the present alloy because of the 0.3% Si content) has even poorer creep resistance. Examples C-D also have poor creep resistance.

The examples within the scope of the present alloy (Ex. l-6) combine high strength with improved creep resistance. The TYS is substantially the same as Ex. A and B. The Creep is reduced by 50% or more.

A sample of an alloy within Gann et al. was permanent mold cast as would be the case with a forging alloy. The composition was 2% A1, 0.8% Si, 0.5% Mn, and the balance Mg. Such alloy had 5.5% E, 6 TYS, 18 TS and 0.16 Creep.

While the creep resistance of the lower aluminum alloy was even better than the alloys within the present invention, this improvement is gained at the expense of strength. Even in the die cast condition the strength values of the lower aluminum'alloys are on the order of 20 percent less than those of the alloy of the present invention. Furthermore, additional tests showed that these comparative alloys had poorer castability, i.e., more difficult to fill the mold and had castings with poorer surface finish. Also corrosion studies indicated that the alloys of the present invention were far superior in corrosion resistance to the comparative lower aluminum alloys, i.e., those within the scope of Gann et al.

Thus the alloy of the present invention possesses a combination of properties heretofore unknown in the prior art alloys.

To show the criticality of processing to achieve this combination of properties, an alloy having the composition of Example 4 was permanent mold cast and the cast product properties tested. Those properties were 5% E, 10 TYS, 22 TS and 0.20% C. In comparison to the die cast product of the same composition (Example 4), the die cast article has substantially improved strength.

What is claimed is:

1. An alloy consisting essentially by weight of 4.1 per cent aluminum, 0.52 per cent silicon, and 0.39 per cent manganese, the balance being magnesium.

2. A method which comprises die casting an alloy composition consisting essentially by weight of 4.1 per cent aluminum, 0.52 per cent silicon, and 0.39 per cent manganese, the balance being magnesium.

* i t s a: 

2. A method which comprises die casting an alloy composition consisting essentially by weight of 4.1 per cent aluminum, 0.52 per cent silicon, and 0.39 per cent manganese, the balance being magnesium. 